This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] where
we do not need to move the position of [[maybe_unused]] within
declarations.
Notes:
- [[maybe_unused]] is a standard feature of C++17.
- The compiler is far more lenient about the placement of
__attribute__((unused)) than that of [[maybe_unused]].
I'll follow up with another patch to finish up the rest.
This patch fixes the following error:
```
llvm/lib/Support/TextEncoding.cpp:274:11: error: cannot initialize a variable of type 'char *' with an rvalue of type 'const char *'
274 | char *Input = InputLength ? const_cast<char *>(Source.data()) : "";
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
These are identified by misc-include-cleaner. I've filtered out those
that break builds. Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.
